How to mark some problems in code

Hi,
Suppose you are looking at code and you spot some flaws. Like duplication or say some better way to write the code. Like the code is too complex with many if conditions.
But right now you dont have time to fix all of these so instead you would like to mark or jot down notes somewhere in the code so that you and someone else can also have a look at it. What are the ways in which one can do this.
I can think of one way like making a TODO. Are there any other better ways ?